Many particulate materials are produced and processed in aqueous media. Before they are sold to customers or further processed, it is often necessary to remove the water. Dewatering can be achieved by either mechanical methods (e.g., filtration and centrifugation) or thermal drying. In general, the former is cheaper than the latter. However, mechanical dewatering becomes inefficient with finer particles. Dewatered products contain high moistures, often requiring thermal drying to meet specifications.
In a given mechanical dewatering process, bulk of the water is removed rather quickly. What is difficult to remove is the water adhering to the surface of the particulate material. Thus, the amount of the residual water left in the product is approximately proportional to its surface area. For a given material, specific surface area is inversely proportional to the square of its particle size. Therefore, the residual moistures in filtered products increase accordingly with decreasing particle size. A more quantitative explanation for the difficulty in dewatering fine particles by filtration may be given by the Laplace equation:                                           Δ            ⁢                          xe2x80x83                        ⁢            p                    =                                    2              ⁢                              xe2x80x83                            ⁢              γ              ⁢                              xe2x80x83                            ⁢              cos              ⁢                              xe2x80x83                            ⁢              θ                        r                          ,                            [        1        ]            
in which xcex94p is the pressure of the water inside a capillary (formed between the particles present in a filter cake), r is the capillary radius, xcex3 is the surface tension of water, and xcex8 is the contact angle of the particles in the cake. The contact angle is a measure of the hydrophobicity (water-hating property) of the particles. Eq. [1] shows that the pressure required to blow the water out of a capillary increases with decreasing capillary radius. Considering that finer particles form smaller capillaries, one can see the difficulty in dewatering fine particles. With a given filter cake, which consists of particles of different sizes, there must be a distribution of capillaries of various radii. At a given pressure drop applied across a filter cake, it would be difficult to blow the water out of the capillaries whose radii are below certain critical value. Thus, the number of capillaries, whose radii are below the critical radius, should determine the final cake moisture.
Various polymeric flocculants are used to enlarge the particle size and, hence, minimize the number of smaller capillaries. Electrolytic coagulants can also be used to enlarge particles. Groppo and Parekh (Coal Preparation, 1996, vol. 17, pp. 103-116) showed that fine coal dewatering improves considerably in the presence of divalent and trivalent cations. They found this to be the case when using cationic, anionic and nonionic surfactants.
Eq. [1] suggests also that capillary pressure should decrease with decreasing surface tension and increasing contact angle. Various surfactants are used to decrease the surface tension. Most of the dewatering aids used for this purpose is ionic surfactants with high hydrophile-lipophile balance (HLB) numbers. Sodium laurylsulfate and sodium dioctylsulfosuccinate, whose HLB numbers are 40 and 35.3, respectively, are typical examples. Singh (Filtration and Separation, March, 1977, pp. 159-163) suggested that the former is an ideal dewatering aid for coal because it does not adsorb on the surface, which in turn allows for the reagents to be fully utilized in lowering surface tension. The U.S. Pat. No. 5,346,630 teaches a method of pressure spraying a solution of a dewatering aid from a position within the filter cake forming zone of a filter just prior to the disappearance of the supernatant process water. This method, which is referred to as torpedo-spray system, ensures even distribution of the dewatering aid without becoming significantly diluted by the supernatant process water.
It is well known that high HLB surfactants can actually cause an increase in moisture in dewatering hydrophobic materials such as coal. Due to the high polarity of its head group, high HLB surfactants adsorb on hydrophobic surfaces with inverse orientation, i.e., with hydrocarbon tails in contact with the surface and the polar heads pointing toward the aqueous phase. Such an adsorption mechanism should decrease the hydrophobicity and, hence, cause an increase in cake moisture. Most of the flocculants used as dewatering aids also dampen the hydrophobicity, and cause an increase in moisture.
There are several U.S. patents, which disclosed methods of using low HLB surfactants as dewatering aids. The U.S. Pat. Nos. 4,447,344 and 4,410,431 disclosed methods of using water insoluble nonionic surfactants with their HLB numbers in the range of 6 to 12. These reagents were used together with reagents (hydrotropes) that are capable of keeping the surfactants in solution or at the air-water interface rather than at the solid-liquid interface, so that they can be fully utilized in lowering surface tension. The advantage of using low HLB surfactants may be that unlike the high HLB surfactants they do not have the deleterious effects of hydrophobicity dampening.
The U.S. Pat. No. 5,670,056 teaches a method of using non-ionic low HLB surfactants and polymers as hydrophobizing agents that can increase the contact angle above 65xc2x0 and, thereby, reduce the cake moisture. Monounsaturated fatty esters, fatty esters whose HLB numbers are less than 10, and water-soluble polymethylhydrosiloxanes were used as hydrophobizing agents. The fatty esters were used with or without using butanol as a carrier solvent for the low-HLB surfactants. This invention disclosure lists a group of particulate materials that can be dewatered using these reagents. These include coals, clays, sulfide minerals, phosphates, metal oxide minerals, industrial minerals and waste materials, most of which are hydrophilic. The use of the low HLB surfactants disclosed in the U.S. Pat. No. 5,670,056 may be able to increase the contact angles of the materials that are already hydrophobic but not for the hydrophilic particles.
The U.S. Pat. No. 2,864,765 teaches a method of using a polyoxyethylene ehter of a hexitol anhydride partial long chain fatty acid ester, functioning alone or as a solution in kerosene. However, the disclosure does not mention that the nonionic surfactant increases the hydrophobicity of moderately hydrophobic particles. Furthermore, the compounds disclosed are essentially not adsorbed upon the solid surface of the ore particles and remain in the filtrate, as noted in the U.S. Pat. No. 4,156,649. In the latter patent and also in the U.S. Pat. No. 4,191,655, methods of using linear or branched alkylethoxylated alcohols as dewatering aids were disclosed. They were used in solutions of hydrocarbon solvents but in the presence of water-soluble emulsifiers such as sodium dioctylsulfosuccinate. As has already been discussed, the use of such a high HLB surfactant can dampen the hydrophobicity and cause an increase in moisture.
The U.S. Pat. No. 5,048,199 disclosed a method of using a mixture of a non-ionic surfactant, a sulfosuccinate, and a deforming agent. The U.S. Pat. No. 4,039,466 disclosed a method of using a combination of nonionic surfactant having a polyoxyalkylene group and an anionic surfactant. The U.S. Pat. No. 5,215,669 teaches a method of using water-soluble mixed hydroxyether, which is supposed to work well on both hydrophobic (coal) and hydrophilic (sewage sludge) materials. The U.S. Pat. No. 5,167,831 teaches methods of using non-ionic surfactants with HLB numbers of 10 to 14. This process is useful for dewatering Bayer process alumina trihydrate, which is hydrophilic. The U.S. Pat. No. 5,011,612 disclosed methods of using C8 to C20 fatty acids, fatty acid precursors such as esters or amides, or a fatty acid blend. Again, these reagents are designed to dewater hydrophilic alumina trihydrate.
The U.S. Pat. No. 4,206,063 teaches methods of using a polyethylene glycol ether of a linear glycol with its HLB number in the range of 10 to 15 and a linear primary alcohol ethoxylate containing 12 to 13 carbon atoms in the alkyl moiety. These reagents were used to dewater mineral concentrates in conjunction with hydrophobic alcohols containing 6 to 24 carbon atoms. The composition of this invention was preferably used in conjunction with polymeric flocculants. Similarly, the U.S. Pat. No. 4,207,186 disclosed methods of using a hydrophobic alcohol and a non-ionic surfactant whose HLB number is in the range of 10 to 15.
It is well known that oils can enhance the hydrophobicity of coal, which is the reason that various mineral oils are used as collectors for coal flotation. The U.S. Pat. No. 4,210,531 teaches a method of dewatering mineral concentrates using a polymeric flocculant, followed by a combination of an anionic surfactant and a water-insoluble organic liquid. The use of flocculant and ionic surfactants may be beneficial in dewatering, but they could dampen the hydrophobicity of the particles and, hence, adversely affect the process. The U.S. Pat. No. 5,256,169 teaches to treat a slurry of fine coal with an emulsifiable oil in combination with an elastomeric polymer and an anionic and nonionic surfactant, dewatering the slurry and drying the filter cake, where the oil reduces the dissemination of fugitive dusts. The U.S. Pat. No. 5,405,554 teaches a method of dewatering municipal sludges, which are not hydrophobic, using water-in-oil emulsions stabilized by cationic polymers. The U.S. Pat. No. 5,379,902 disclosed a method of using heavy oils in conjunction with two different types of surfactants, floating the coal-emulsion mixture, dewatering the flotation product and drying them for reconstitution. The U.S. Pat. No. 4,969,928 also teaches a method of using heavy oils for dewatering and reconstitution.
The U.S. Pat. No. 4,770,766 disclosed methods of increasing the hydrophobicity of oxidized and low-rank coals using additives during oil agglomeration. The main objective of this process is to improve the kinetics of agglomeration and ultimately the separation of hydrophilic mineral matter from coal. The additives disclosed in this invention include a variety of heavy oils and vegetable oils, alcohols containing 6 or more carbon atom, long-chain fatty acids, etc. When these additives were used, the product moisture was lower than would otherwise be the case. However, the process requires up to 300 lb/ton of additives and uses very large amounts (45 to 55% by volume of a coal to be cleaned) of an agglomerant, which is selected from butane, hexane, pentane and heptane.
The U.S. Pat. No. 5,458,786 disclosed a method of dewatering fine coal by displacing water from the surface with a very large amount of liquid butane. The spent butane is recovered and recycled. The U.S. Pat. No. 5,587,786 teaches methods of using liquid butane and other hydrophobic liquids for dewatering other hydrophobic particles.
A co-pending U.S. patent application, (whose filing number is not known at the time of filing the present application), discloses a method of improving dewatering fine particulate materials by hydrophobizing a fine particulate material using a suitable high HLB surfactant and then further enhancing its hydrophobicity using a well-defined nonionic surfactants of low HLB numbers. However, some of the reagents disclosed in this invention are costly.
It is an object of the present invention to provide novel methods of decreasing the moisture of fine particulate materials during mechanical dewatering processes such as vacuum and pressure filtration and centrifugation.
Another important objective of the invention is the provision of improving the rate at which water is removed so that given dewatering equipment can process higher tonnages of particulate materials.
An additional objective of the present invention is the provision of novel fine particle dewatering methods that can reduce the moisture so low that no thermal drying is necessary.
Still another object of the instant invention is the provision of a novel dewatering method that creates no adverse effects on up- and downstream processes when the water removed from the dewatering processes disclosed in the present invention is recycled.
Yet another object of the invention is the provision of methods of controlling the frothing properties of the flotation product.
Perhaps the most important object of the instant invention is to achieve all of the above objects using low-cost affordable dewatering aids that have no harmful effects on the environment and the human health.
It is the object of this invention to provide an efficient method of dewatering fine particulate materials. This is achieved by destabilizing the water on the surface of the particles to be dewatered by rendering the surface substantially hydrophobic. The particles are hydrophobized normally in two steps. Initially, surfactants of high hydrophile-liphophile balance (HLB) numbers are used to render a particulate material moderately hydrophobic. The material is subsequently treated with a lipid, which is a naturally occurring hydrophobic substance, to further enhance its hydrophobicity close to or above the water contact angle of 90xc2x0. This will greatly weaken the bonds between the water molecules and the surface of the particulate material and, thereby, xe2x80x98liberatexe2x80x99 the surface water. The liberated surface water is then removed from the particulate material by using various mechanical dewatering devices.
The key to the methods of dewatering described in the present invention disclosure is the hydrophobicity enhancement step. According to the Laplace equation, a relatively small increment in hydrophobicity (above the level that can normally be achieved using a high HLB surfactant in the first hydrophobization step) can bring about a large decrease in capillary pressure and, hence, a large decrease in surface moisture. The initial hydrophobization step may be omitted, if the particulate material is naturally hydrophobic or has been sufficiently hydrophobized in an upstream process, e.g., flotation, preceding dewatering. However, the particles must remain reasonably hydrophobic at the time of the hydrophobicity enhancement step. Otherwise, the dewatering aids added in this step does not adsorb on the surface and fails to enhance its hydrophobicity.
The lipids used in the second hydrophobization step of the instant invention are insoluble in water; therefore, they are used as solutions in appropriate solvents, which include but not limited to light hydrocarbon oils and short-chain alcohols. When used in conjunction with an appropriate solvent, lipid molecules may act as nonionic surfactants that can greatly enhance the hydrophobicity of the particulate material to be dewatered. Since lipids are naturally occurring reagents, their use offers a low cost means of improving mechanical dewatering processes.
The dewatering methods disclosed in the instant invention are capable of not only reducing the final cake moistures but also of increasing the kinetics of dewatering substantially. By virtue of the latter, the instant invention can greatly increase the throughput of a dewatering device. Furthermore, the dewatering aids of the present invention have the characteristics of anti-forming agents, which is very important for processing the particulate materials produced from flotation processes. Also, most of the reagents added as dewatering aids and blends thereof adsorb on the surfaces of minerals and coal so that the plant water does not contain significant amounts of residual reagents.
The difficulty in removing water from the surface of fine particles may be attributed to the fact that water molecules are held strongly to the surface via hydrogen bonding. It is possible to break the bonds and remove the water by subjecting the wet particles to intense heat, high-pressure filters and high-G centrifuges. However, the use of such brute forces entails high energy costs and maintenance problems. A better solution would be to destabilize the surface water by appropriate chemical means, so that it can be more readily removed by using mechanical dewatering devices with minimum energy and maintenance requirements.
The state of the water adhering to a surface may be best represented by the hydrophobicity (water-hating property). The stronger the hydrophobicity, the weaker the bonds between the water and the surface. Therefore, the key to finding appropriate chemical means to destabilize surface water is to increase the hydrophobicity of the particles to be dewatered. A more traditional measure of surface hydrophobicity is water contact angle. In the cessile drop technique, contact angles are measured by placing droplets of water on the surface of the solids of interest. The contact angle, which is measured through the aqueous phase, increases with increasing hydrophobicity.
More recently, scientists developed methods of measuring the forces between two macroscopic surfaces approaching to each other in water. They discovered a hitherto unknown attractive force, which is generally referred to as hydrophobic force. Many researchers showed that the new attractive force is 10 to 100 times stronger than the omnipresent van der Waals force. Yoon and Ravishankar (J. Colloid and Interface Science, vol. 179, p. 391, 1996) showed that the hydrophobic force increases sharply when the contact angles of two interacting mica surfaces approach 90xc2x0. According to Eq. [1], capillary pressure becomes negative at contact angles above this value. Thus, if one can increase the hydrophobicity of a particulate material to the extent that its water contact angle exceeds 90xc2x0, water should be removed spontaneously. This may be achieved using appropriate surfactants. According to Flinn, et al. (Colloids and Surfaces A, vol. 87, p. 163, 1994), the hydrocarbon tails of octadecylchlorosilane begin to stand up vertically and form a close-packed monolayer on the surface of silica at a contact angle close to or above 90xc2x0. Also, Yoon and Ravishankar observed long-range hydrophobic forces only when close-packed monolayers were formed on mica surfaces. It appears, therefore, that the key to achieving spontaneous dewatering may be to find appropriate surfactants or combinations thereof that can form close-packed monolayers of hydrophobes on the surfaces of the particles to be dewatered.
In the instant invention, the particulate materials in a slurry are hydrophobized in two steps. In the first step, an appropriate surfactant is added to the slurry, so that it can adsorb on the surface of the particles and render them moderately hydrophobic. For hydrophilic particles such as silica and clay, ionic surfactants of high HLB numbers may be used for the initial hydrophobization. For sulfide minerals, short-chain thiols may be used. These reagents adsorb on the surface with their polar heads in contact with the surface and their hydrocarbon tails directed toward the aqueous phase. For naturally hydrophobic materials of moderate hydrophobicity, hydrocarbon oils and short-chain alcohols may be used to enhance the hydrophobicity. If the particles are sufficiently hydrophobic, no reagents may be necessary in the first hydrophobization step. In the second step, a lipid dissolved in an appropriate solvent or a mixture of solvents is added to the slurry to further increase the hydrophobicity of the particulate materials, so that the surface water can be removed more readily by mechanical dewatering processes of low energy consumption.
As a result of the first hydrophobization step, the contact angle of the particulate material to be dewatered may be increased to the range of 25xc2x0 to 60xc2x0. It is difficult, but not impossible, to obtain contact angles above this range using a high HLB surfactant alone. High HLB surfactants and thiols adsorb only on specific surface sites. The population of the surface sites, at which the adsorption can occur, is usually well below what is needed to form a close-packed monolayer of the adsorbed surfactant molecules. The reagents added in the second hydrophobization step, i.e., the lipids dissolved in appropriate solvents, may adsorb in between the sparsely populated hydrocarbon tails of the high HLB surfactants and thiols, so that the surface is more fully covered by a close-packed monolayer of hydrophobes. This will increase the contact angle over 60xc2x0 and more desirably close to or over 90xc2x0, so that water can be readily removed from the capillaries formed between finer particles.
Although the Laplace equation suggests that contact angle must exceed 90xc2x0 for spontaneous dewatering, increasing contact angles close to but not exceeding this value can bring about sufficient advantages. A close examination of Eq. [1] reveals that an increase in contact angle beyond what can be achieved in the first hydrophobization step can bring about a substantial decrease in capillary pressure and, hence, a reduction in cake moisture. Consider a case where contact angle is increased from zero to 60xc2x0 in the first hydrophobization step. This should decrease capillary pressure by only one half. If the angle is further increased from 60xc2x0 to 85xc2x0 in the second hydrophobization step, the capillary pressure decreases further by 5.7 times. This is a substantial gain that can be achieved by a seemingly a modest increment in contact angle. Thus, the second hydrophobization step disclosed in the instant invention offers a highly efficient means of substantially lowering capillary pressures and, thereby, achieving very low cake moistures. A co-pending U.S. Patent Application (whose number is not known at the time of the current application) filed by the inventor of the instant invention also discloses the advantages of incorporating a second hydrophobization step. In this co-pending application, well-defined low HLB surfactants are used as the hydrophobicity-enhancing reagent. However, many of the low HLB surfactants are considerably more expensive than the lipids disclosed in the instant invention.
Lipids are naturally occurring organic molecules that can be isolated from plant and animal cells (and tissues) by extraction with nonpolar organic solvents. Large parts of the molecules are hydrocarbons (or hydrophobes); therefore, they are insoluble in water but soluble in organic solvents such as ether, chloroform, benzene, or an alkane. Thus, the definition of lipids is based on the physical property (i.e., hydrophobicity and solubility) rather than by structure or chemical composition. Lipids include a wide variety of molecules of different structures, i.e., triacylglycerols, steroids, waxes, phospholipids, sphingolipids, terpenes, and carboxylic acids. They can be found in various vegetable oils (e.g., soybean oil, peanut oil, olive oil, linseed oil, sesame oil), fish oil, butter, lard and tallow. Animal fats and vegetable oils are the most widely occurring lipids. Although fats and oils appear different, that is, the former are solids and the latter are liquids at room temperature, their structures are closely related. Chemically, both are triacylglycerols; that is, triesters of glycerol with three long-chain carboxylic acids. They can be readily hydrolyzed to fatty acids. Corn oil, for example, can be hydrolyzed to obtain mixtures of fatty acids, which consists of 35% oleic acid, 45% linoleic acid and 10% palmitic acid. The hydrolysis products of olive oil, on the other hand, consist of 80% oleic acid. Waxes can also be hydrolyzed, while steroids cannot. Vegetable fats and oils are usually produced by expression and solvent extraction or a combination of the two. Pentane is widely used for solvent, and is capable of extracting 98% of soybean oil. Some of the impurities present in crude oil, such as free fatty acids and phospholipids, are removed from crude vegetable oils by alkali refining and precipitation. Animal oils are produced usually by rendering fats.
In the instant invention, the lipids may act as natural surfactants that can enhance the hydrophobicity of the particles to be dewatered. Each triacylglycerol, for example, consists of one head group, i.e., glycerol, and three hydrocarbon tails. For steroids, hydroxyl groups may act as polar head, while the ester linkages serve as the head groups with waxes. They may act effectively as nonionic surfactants of low hydrophile-lipophile balance (HLB) numbers. The HLB numbers of soybean oil and corn oil are 6 and 8, respectively, while that of castor oil is 14. They may adsorb in between or on top of the hydrocarbon chains of the surfactants and thiols that are present on the surface of fine particles as a result of the first hydrophobization step and, thereby, enhance the hydrophobicity.
Since the lipids have low HLB numbers, they may be used as solutions of appropriate solvents including but not limited to short-chain alcohols and light hydrocarbon oils. Typically, one part by volume of a lipid, which may be termed as active ingredient(s), is dissolved in two parts of a solvent before use. The two may be mixed in different ratios. As an example, three parts of an active ingredient may be mixed with one part of a solvent. In another, one part of an active ingredient may be mixed with 20 parts of a solvent.
The first hydrophobization step described above may be omitted, if a particulate material is naturally hydrophobic, or has acquired a moderate hydrophobicity in a process preceding dewatering step. For example, selected mineral (or coal) constituents of an ore (or coal) are selectively hydrophobized using appropriate reagents (e.g., high HLB surfactants, thiols, light hydrocarbon oils and short-chain alcohols) and floated away from hydrophilic mineral constituents as a means of separation and upgrading. The particulate material to be dewatered must be moderately hydrophobic for the second hydrophobization step disclosed in the instant invention to work. Otherwise, the hydrophobic lipids disclosed cannot adsorb on the surface via hydrophobic attraction and enhance its hydrophobicity. Frequently, the naturally hydrophobic materials or mineral concentrates become considerably less hydrophobic by the time they reach the dewatering step due to superficial oxidation, aging, or exposure to plant water containing hydrophilic polymers. In such cases, they may be re-hydrophobized using the high HLB surfactants and other reagents noted above before adding the reagents identified in the instant invention for the second hydrophobization step.
It may be useful to note here that mineral and coal concentrates obtained by flotation is not hydrophobic enough to be dewatered efficiently. The reason is that the thermodynamic requirement for bubble-particle adhesion, which is a prerequisite for flotation, is that contact angle be larger than zero, while the thermodynamic requirement for spontaneous dewatering is 90xc2x0, as discussed above. Therefore, the second hydrophobization step is essential to reduce the cake moisture beyond the levels usually achieved using the currently available dewatering aids and methods. The use of lipids in the second hydrophobization step provides a low-cost means of increasing the contact angle close to or above 90xc2x0.
For a given particulate material, parts of the surface must be more hydrophobic than the rest. When using a lipid as dewatering aid, most of the molecules may adsorb on the more hydrophobic parts of the surface, thereby increasing the packing density of hydrophobes on the surface and further increasing its hydrophobicity. The driving force for the adsorption mechanism may be one of hydrophobic attraction. On the other hand, some of the lipid molecules may adsorb on less hydrophobic parts of the surface, with the oxygens in the head groups in contact with the less hydrophobic parts of the surface, possibly via acid-base interactions, while the hydrocarbon tails are pointed toward the aqueous phase. The net result of this adsorption mechanism would be a conversion of the less hydrophobic parts of a surface to more hydrophobic ones. Both of these adsorption mechanisms, i.e., one based on hydrophobic interaction and the other based on acid-base interactions, should help increase the surface hydrophobicity substantially, with its contact angle approaching or exceeding 90xc2x0.
The light hydrocarbon oils used as solvents for lipids may also adsorb on the surface of the particulate material to be dewatered via hydrophobic interaction, and further enhance its hydrophobicity. In effect, the lipid molecules may act as nonionic surfactants and help spread the light hydrocarbon oils on the surface by modifying the interfacial tensions involved. The lipid molecules should increase the interfacial tension at the solid/water interface, as a consequence of rendering the surface more hydrophobic, while causing a decrease in the interfacial tensions at the oil/water and solid/oil interfaces. Improved spreading of the light hydrocarbon oil should contribute to enhancing the surface hydrophobicity close to or above 90xc2x0. Furthermore, all of the reagents used in the present invention may also serve as surface tension lowering agents. The surface tensions of the lipids, hydrocarbon oils and short-chain alcohols are substantially lower than that of water. Their presence at the air-water interface by virtue of their hydrophobicity should reduce the surface tension, and thereby help reduce cake moistures according to the Laplace equation.
Many different samples were used for dewatering tests. These include fine silica, kaolin clay from middle Georgia (60% finer than 2 xcexcm), various coal samples from different sources, and sulfide mineral concentrates. A hydrophilic material such as silica and kaolin was hydrophobized in two steps: first using a high HLB surfactant to render the surface moderately hydrophobic and then using a lipid to further enhance its hydrophobicity. Since lipids are insoluble in water, they were used after dissolution in suitable solvents. When sulfide mineral concentrates were received from abroad, they were superficially oxidized and became hydrophilic. As a means of regenerating fresh hydrophobic surfaces, they were re-floated using a thiol collector and methylisobutyl carbinol (MIBC) as a frother. This was necessary because lipids do not adsorb on hydrophilic surfaces.
Some of the coal samples were used as received. Most of the tests were conducted, however, after re-flotation using standard flotation reagents such as kerosene and MIBC. When a sample became hydrophilic due to aging or superficial oxidation during transportation, it was wet-ground in a ball mill for a short period of time to remove the oxidation products and regenerate fresh, moderately hydrophobic surfaces. Lipids adsorb on the surface and enhance its hydrophobicity. To minimize the problems concerning oxidation, some of the tests were conducted using coarse dense-medium products. They were crushed, pulverized, wet-ground in a ball mill, and floated using kerosene and MIBC. The float product was placed in a container and agitated. A known volume of the slurry was removed and transferred to an Elenmeyer flask. After adding known amounts of reagent(s), the flask was hand-shaken for 2 minutes. The conditioned slurry was poured into a filter to initiate a dewatering test. After a preset drying cycle time (usually 2 minutes), the product was removed from the filter, dried in an oven for overnight, and then weighed to determine the cake moisture. In each test, cake formation time and cake thickness were recorded. The cake formation time is defines as the time it takes for bulk of the water is drained and a cake is formed on a filter medium. For vacuum filtration, a 2.5-inch diameter Buchner funnel with a medium porosity glass frit was used. When it was desired to conduct tests at large cake thicknesses, the height of the Buchner filter was extended. For pressure filtration, a 2.5-inch diameter air pressure filter with a cloth fabric medium was used. It was made of Plexiglas so that the cake formation time could be determined by visual observation.